The present invention relates to optical communications, and more particularly to an optical cross connect switch using programmable multiplexers and demultiplexers to switch optical wavelength division multiplexed (WDM) channels among multiple inputs and outputs in an optical communication system.
The transmission capacity of fiber-optic communication systems has increased significantly with the wavelength division multiplexing technique. In a WDM system, multiple channels, where each channel is differentiated by the use of a different wavelength of light, carry modulated optical signals in a single optical fiber. Optical multiplexers are used at the transmitter to combine all the optical channels into the fiber for transmission, and optical demultiplexers are used at the receiver to separate the optical channels for detection.
In an optical network, network traffic can be routed from a source to a destination via one or more intermediate nodes, each of which is connected to a plurality of neighboring nodes. Accordingly, each intermediate node requires some switching or cross connection capability to select an appropriate neighboring node in order to route the traffic towards the desired destination. An intermediate node can operate electronically, by (a) terminating wavelength channels at a receiver endpoint, (b) switching the traffic with electronic means, and (c) originating the wavelength channels at transmitter points. Alternatively, the switching nodes can operate transparently, routing the individual optical channels without opto-electronic conversion. Regardless, intermediate nodes are required to switch incoming wavelength channels from an input port to a desired output port, so that a channel originating at an upstream node can pass through the intermediate node enroute toward its downstream node destination.
FIG. 1 illustrates the topology of a prior art optical communications system network node with conventional switching functionality. The network node has several input transmission ports 110, 120 and 130 carrying multiplexed optical channels and several output transmission ports 154, 164 and 174 carrying the outbound multiplexed optical channels. Each input multiplexed channel, e.g. the channel on input transmission port 110, is demultiplexed by a demultiplexer 112, separating the optical channels on distinct demultiplexer ports such that optical channel with wavelength xcex1 appears on demultiplexer port 114-1, the optical channel with wavelength xcex2 appears on demultiplexer port 114-2, and so on, through optical channel with wavelength xcexN which appears on demultiplexer port 114-N. The demultiplexed optical channels from all the input ports 110, 120 and 130, having been demultiplexed by demultiplexers 112, 122 or 132, are introduced to the input ports 180-1 through 180-3N of a cross connect switch 140, which independently routes traffic at each input port towards any one of the cross connect switch output ports 190-1 through 190-3N. These switch output ports are connected to the multiplexer ports 150-1 through 150-N, 160-1 through 160-N and 170-1 through 170-N of multiplexers 152, 162, and 172, respectively, which combine or multiplex each of the channels onto one of the output transmission ports 154, 164 or 174. In order to connect each input channel to the appropriate one of the output transmission ports 154, 164 or 174, cross connect 140 must be arranged to interconnect each of its input ports 180-1 through 180-3N with the appropriate one of its output ports 190-1 through 190-3N.
Cross-connect switch 140 can be implemented by either electronic or optical switching fabrics. The number of cross-connect switch ports has to be as large as the number of input transmission ports times the number of optical channels received via each such port, which typically requires the number of cross-connect switch ports to be in the hundreds or higher.
Instead of the arrangement of FIG. 1, the alternative prior art architecture shown in FIG. 2 may be used. All the input optical channels arriving at input transmission ports 210, 220 and 230 are demultiplexed in demultiplexers 212, 222 and 232 in a similar fashion to the arrangement of FIG. 1. However, switching is performed by a cluster of small cross connect switches 240-1, 240-2, 240-N, each switch handling a single optical wavelength channel. For example, all of the demultiplexer ports 214-1, 224-1 and 234-1 carrying channels with wavelength xcex1 are switched by cross connect switch 240-1 to the proper multiplexer port 250-1, 260-1, or 270-1 of multiplexers 252, 262 and 272, respectively. Likewise, for a different wavelength 2, demultiplexer ports 214-2, 224-2 and 234-2 carry channels with that wavelength to cross connect switch 240-2 and thence to the proper multiplexer ports 250-2, 260-2, or 270-2. This arrangement requires an individual cross connect for each of the optical channels. The port count of each cross connect switch is determined by the number of input transmission ports. Since the trend in optical communication systems is to increase the number of optical channels from a few hundred today to over a thousand in the future, the arrangement of FIG. 2 does not scale well and will undesirably require a massive fiber interconnect patch panel.
In accordance with the present invention, an optical cross-connect switch is based on and uses the programmable optical multiplexer/demultiplexer as described in co-pending application Ser. No. 09/944,800 filed on Sep. 31, 2001 and assigned to the same assignee as the present application. As described in the aforementioned co-pending application, a programmable optical demultiplexer is arranged to receive a multiplexed optical signal containing a plurality of separate channels, each with an associated wavelength, and independently assign each input optical channel to a desired output port. Likewise, a programmable optical multiplexer is arranged to receive a plurality of separate optical channels, each with an associated wavelength, and combine the different wavelengths into a single multiplexed optical signal that is made available at the multiplexer output port.
In accordance with one embodiment of the present invention, an optical cross connect switch includes a programmable demultiplexer placed on every input transmission port, a programmable multiplexer placed on every output transmission port, and a linking fiber between every programmable demultiplexer and multiplexer in the system. The programmable demultiplexers and multiplexers handle internally all the optical communication channels, and can route any specified optical channel from an input port to the desired output port. Advantageously, the arrangement can send several input channels to the same output port.
In accordance with other embodiments of the present invention, an optical cross connect switch is arranged to both add and drop WDM channels. The added and/or dropped channels are coupled to/from the previously mentioned programmable multiplexers/demultiplexers via additional linking fibers, through the use of additional multiplexers and demultiplexers, which may be conventional or programmable.
In accordance with yet other embodiments of the present invention, either the programmable multiplexers or programmable demultiplexers are replaced by passive combiners or splitters, and concentrators and/or distributors are incorporated in order to reduce the number of transmitters and/or receivers required in the cross connect implementation, and to thereby allow the transmitters and/or receivers that are used to be shared.
The cross connect switch arrangement of the present invention efficiently handles a large optical communication channel count, offers a scalable cost-effective solution for expanding switching capacity, and reduces the fiber interconnection count between switch modules. When a new optical communication system is added to the node, it requires only that a programmable multiplexer and demultiplexer be placed on its output and input fibers, respectively, and fibers connected to its neighboring programmable multiplexers and demultiplexers. The arrangement thus scales proportionally with the number of input line systems to the switch, regardless of the number of optical channels, and provides a pay as you grow solution.